My fish are dying en mass after 50% water change?

Don't beat yourself up, we've all had an unforeseen disaster at one time or another, at least if we've been at it any amount of time.

I have, in an emergency(3 year old dumped about 2 ounces of flake food in a 20 gallon tank) done a 90% change successfully, but you can never forget to:

A) dechlorinate if you're on city water

B) match the temperature of the new water as closely as possible

Temperature, pH difference or something on the roof contaminating the water are the only things I can think of in your case. If there's much of a pH change once it's in your tank, small water changes might be okay but a large one could mess you up. Check the pH in the tank and the pH in your source water, check your roof and see if there's exposed copper or anything out of the ordinary.(A dead bird could do it)

D is correct, over 90% of your cycle bacteria is in the filter media, almost none id free floating. Nitrosoma and nitrobacter bacterias require a surface to grow on and a flow of food and oxygen.

I know what you did and its something we all do as beginners. So don't feel too bad.

By cleaning the gravel and stirring up the muck you exposed bacteria, poo and waste to fresh oxygenated water and so caused a huge release of hydrogen sulphide gas trapped in the gravel and this is very toxic.

Also the disturbed muck will be decaying fast now because you've exposed it to oxygen and producing lots of ammonia and nitrite which is also toxic. So you caused what is called a "Mini-cycle".

Only stir up a small portion of the gravel at any one time you do a water change, say 25%. Ensure you oxygenate the water with a air stone and then use Prime sold by Seachem over the next few days as this is the best water conditioner in the world as it renders non-toxic ammonia _and_ nitrite. (you also get more for your money)

Keep your gravel substrate thin, less than 1" thick and it'll help prevent pockets of trapped H2S from happening.

Tip: if you have gravel which has turned black then that is where the H2S gas is coming from.

You can eliminate this safely by adding a Waste Control bacteria every week. It also keeps your fish and substrate healthy and much less likely to suffer from bacteria diseases which attack the mouth or fins.

Alternatively set up a Walstad tank. This is an amazing low cost way to keep fish and the bacteria in the soil keeps the bad bacteria away.

I hope you weren't too upset at your losses.

it could be a couple of things:

1. the chlorine in the water

2. the new water has changed the tank temperature out of a safe range

in the future theres no reason to do a 50% change, i stick to 20-30%

you dont have to cycle it again, the bacteria you need is not in the water its in the filter pad and substrate and rock

No the tank is not cycling again. You critically over changed your water and put your fish into shock. A water change of that magnitude is HUGE! Leave the Tank Alone and hope the worst is over.

AND FROM NOW ON, stick to the 20% water changes that you know work.

Best of Luck :)

Did you put water conditioner in as you need to get rid of all the chemicals